The present invention relates to a method of operating a hydraulic brake system.
German patent application No. 43 33 568 discloses a brake system for which the mentioned method is appropriate. The prior art brake system is equipped for brake slip control according to the recirculation principle, for traction slip control on the wheel brakes of the driven wheels and for yaw torque control during cornering. `Active braking` refers to both traction slip control and yaw torque control by brake management because brake management operations of this type may also be effected without the application of the brake pedal. Each brake circuit includes a recirculation cycle, and the return pump is of the self-priming type and has a suction line to the brake line. A separating valve which is being closed during active braking operations is interposed in the brake line between the port of the suction line and the port of the pressure line of the return pump. For a rapid pressure build-up, the brake system includes a high-pressure accumulator on the pressure side of the return pump which is connectable to the brake line by way of an electromagnetic two-way/two-position directional control valve. In the normal case, the return pump aspirates pressure fluid from the low-pressure accumulator connected to the outlet valves of the wheel brakes. The return pump delivers the fluid volume from the low-pressure accumulator into the brake line where it prevails upstream of the inlet valves of the wheel brakes. In contrast to brake slip control, this fluid volume is required during active braking for pressure build-up in the wheel brakes. The wheel brakes concerned are selected according to the basic control strategies, i.e., traction slip control or yaw torque control.
When the low-pressure accumulator is completely exhausted, the return pump can neither aspirate fluid volume, nor supply it into the brake line. Active pressure build-up in the wheel brakes of the brake circuit concerned becomes impossible. When a minimum filling level in the low-pressure accumulator is reached, the electric change-over valve arranged in the suction line to the brake line must be opened so that the return pump can aspirate fluid from the supply reservoir of the brake system by way of the master cylinder. During active braking operations, it is imperative to prevent an operating condition with a completely empty low-pressure accumulator and a closed electric change-over valve. So far, this problem has been overcome by a special travel switch which is provided on the low-pressure accumulator and issues a signal to open the electric change-over valve when the low-pressure accumulator is empty.
However, because additional component parts in a brake system always increase costs and failure of a travel switch involves a safety risk for the operation of the brake system, an object of the present invention is to provide a method of the initially mentioned type which obviates the need for additional components and ensures reliable opening of the electric change-over valve in the suction line between the suction side of the return pump and the brake line when the low-pressure accumulator is empty.